Herons and Egrets are charismatic symbols of wetlands, and they play important roles in how wetlands and other ecosystems function. Tracking the number of herons and egrets in an area can give us a window into the health of the local environment.
Every year since 1989, a dedicated crew of volunteers has fanned out over the northern San Francisco Bay Area to monitor nesting herons and egrets. They count how many nests are in each colony, and estimate the number of young produced in each nest. Long term monitoring projects like this help use understand how ecosystems change over time as humans continue to alter the landscape and climate continues to warm.
Because colonies may vary in size from a few nests to over 100 nests, the best way to represent changes in the number of nests is the percent change in size of each colony. For example, if a colony had 10 nests last year and 15 nests this year, then that would be a 50% increase, and if a colony with 100 nests added those same 5 nests, then that would be a 5% increase. When we average this percent change across all colonies, we can estimate whether the overall breeding population of that species increased (average % change above 0) or decreased (average % change below 0) from last year.
In 2019, we counted 966 Great Egret nests in 26 active colonies. On average, the number of nests in Great Egret colonies increased by 13.6% from 2018. Overall, Great Egret colonies have increased in size during most of this monitoring project. This suggests that the local population of Great Egrets is increasing.
In 2019, 13 Great Egret colonies increased in size, 13 colonies decreased, and 0 colonies remained the same size.Great Egrets nested in 2 colonies where they did not nest in 2018 (Ryer Island, Suisun Bay; Meadowlane Ponds).There were 2 active Great Egret colonies in 2018 that had zero nests in 2019 (Fitch Mountain; Inverness Park).
In 2019, we counted 377 Great Blue Heron nests in 43 active colonies. On average, the number of nests in Great Blue Heron colonies decreased from 2018 by 1.2%. Overall, the number of nests in Great Blue Heron colonies has increased on average over the last several years, although often not by very much. This suggests that the local Great Blue Heron population is generally stable.
In 2019, 13 Great Blue Heron colonies increased in size, 22 colonies decreased, and 10 colonies remained the same size.Great Blue Herons nested in 2 colonies where they did not nest in 2018 (Gold Hill; Ryer Island, Suisun Bay).There were 4 active Great Blue Heron colonies in 2018 that had zero nests in 2019 (Inverness Park; Delta Pond; Spoonbill; Fox Ridge).
In 2019, we counted 240 Snowy Egret nests in 8 active colonies. On average, the number of nests in Snowy Egret colonies increased by 30.6% from 2018.The number of nests in Snowy Egret colonies has generally been stable during our monitoring project, suggesting an overall stable population. However, there have been occasional single year spikes in colony growth, which we don’t yet fully understand.
In 2019, 2 Snowy Egret colonies increased in size, 8 colonies decreased, and 0 colonies remained the same size.Snowy Egrets nested in 1 colony where they did not nest in 2018 (Gold Hill).There were 3 active Snowy Egret colonies in 2018 that had zero nests in 2019 (Wheeler; Congress Valley; Willota Drive).
In 2019, we counted 333 Black-crowned Night-Heron nests in 6 active colonies. On average, the number of nests in Black-crowned Night-Heron colonies decreased from 2018 by 35.5%. Overall, the number of nests in Black-crowned Night-Heron colonies tends to increase and decrease more than the other species we monitor. This means that although the population appears generally stable over the long term, there can be dramati changes in the number of nesting birds from one year to the next.
In 2019, 1 Black-crowned Night-Heron colony increased in size, 8 colonies decreased, and 0 colonies remained the same size.There were 3 active Black-crowned Night-Heron colonies in 2018 that had zero nests in 2019 (Congress Valley; South Kelly; River Montessori).
On this map, upward-pointing triangles indicate colonies that grew in size from 2019 to 2018, downward triangles represent colonies that shrank, and circles represent no change. The size of the triangle represents how much the colony changed. You can zoom in and pan, and hover over each colony to get more information.